Pediatric heart failure cost the United States an estimated $2 billion in 2006 and is increasing in prevalence and cost despite hospital mortality unfortunately remaining constant at 7%. The big-picture goal of our current research program is to attempt to address this problem by adapting a relatively new, highly successful therapy for adult heart failure called cardiac resynchronization therapy (CRT), into a treatment option for children with heart failure. The most critical knowledge gap is identifying which children are most likely to benefit from CRT. The current application addresses this challenge in a population of patients with a common, serious congenital heart defect known as tetralogy of Fallot (TOF). Patients born with tetralogy of Fallot (TOF), the most common cyanotic congenital heart defect, currently undergo surgical repair as children but later develop heart failure as adults. This research will investigate whether the surgical repair predisposes these patients to uncoordinated contraction in the heart as a result of disruption of electrical conduction pathways which occurs during surgery. If this uncoordination (or dyssynchrony) proves to be a significant factor, it can be treated with CRT, thus yielding a new treatment for patients with heart failure due to TOF. We are able to explore the role of dyssynchrony in these patients because we recently developed unique methodology and software to derive indices of dyssynchrony in the heart from standard magnetic resonance images (MRI). We hypothesize that patients with surgically repaired TOF suffer from dyssynchronous contraction in the heart and that dyssynchrony contributes to the development of heart failure and eventual mortality in these patients. We propose to test our hypotheses with the following specific aims: 1) evaluate whether patients with repaired TOF suffer from dyssynchrony when compared to normal subjects 2) evaluate whether dyssynchrony leads to worsening ventricular function over time in patients with repaired TOF and 3) determine whether dyssynchrony is related to mortality in patients with repaired TOF. Completion of this application should therefore improve the health and care of patients with repaired TOF through identifying potentially lifesaving treatment options for them. Additionally, our approach can be used in future work to explore the role of dyssynchrony in patients with other diseases associated with electrical conduction abnormalities such as dilated cardiomyopathy, thus furthering our long-term goal to translate cardiac resynchronization therapy into a treatment option to improve the health of children with heart failure.